MFC 218970,219240:

MFC 218970,219240:
Use umtx_key objects to uniquely identify futexes.  Private futexes in
different processes that happen to use the same user address in the
separate processes will now be treated as distinct futexes rather than the
same futex.  We can now honor shared futexes properly by mapping them to a
AUTO_SHARED umtx_key.  Private futexes use THREAD_SHARED umtx_key objects.

git-svn-id: svn://svn.freebsd.org/base/stable/8@219381 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f